1. Field of the Invention
This invention relates to systems for providing changes in the concentration of solutions. In a primary application the invention relates to the desalinization of salt water.
2. Description of Prior Art
Desalinization by reverse osmosis first became a possibility with the development of suitable membranes in the 1950's. An early reference on the subject is "Seawater Demineralization by Means of an Osmotic Membrane," by S. Loeb and S. Sourirajan in the Advancements in Chemistry Series, 38 p. 117, 1962. Development has followed two paths; the improvement of the integrity and performance of the membrane material itself, and finding suitable packaging and support for the membrane. The membrane material development has aimed at eliminating pin holes, maintaining uniformity and obtaining materials which impede the flow of salt while giving large water flows for pressures in excess of the osmotic pressure.
These problems are considerably augmented for high concentrations of salt, as is found in sea water, where the osmotic pressure is high. Because the process is driven by pressure, confining containers are required for the membranes. Many packing configurations have been used which attempt to provide a large surface area in a given volume in such a way that the membrane is supported and that a flushing action of the brine can occur. Earlier most practical desalinating has been accomplished at low concentrations of salt of the order of 3,000 parts per million. Recently membranes have become commercially available for salt concentrations in the seawater range of 35,000 ppm. These membranes are available from UOP in the spiral wound type and from Dupont and Dow in the hollow fiber type.
Much prior art exists in rotating structures of the centrifuge type. These produce a high acceleration that then acts differently on materials of different density. The primary purpose of a centrifuge is to separate by means of density differences. However, when rotating structures are used in reverse osmosis accelerators, the primary separation occurs in the membrane due to a pressure difference. The density difference is useful to clean the membranes and not to effect a separation.
There are four patents in the prior art which deal with rotating structures accelerating fluids through reverse osmosis membranes and which appear to be the most relevant as background to this invention: U.S. Pat. No. 3,355,382, "Centripetal Acceleration Method and Apparatus," issued to M. G. Huntington is aimed at primarily preventing the excessive concentration of dense brine at the membrane. The membranes, in this patent, are arranged in concentric cylinders in the form of a basket. This configuration provides a limited amount of surface area of the membrane material for the salt water. This limits the conversion efficiency into desalinated water. In addition, this basket configuration of membrane material is difficult to replace. The desalinated water is returned to the axis of rotation after passing through the membrane material. As a result, an additional external source of pressure is required to drive the brine solution through the reverse osmosis membranes.
A similar system is described in U.S. Pat. No. 3,400,074, "Centrifugal Reverse Osmosis for Desalinization," issued to C. A. Grenci. In this patent the reduced concentration mixture, representing desalinated water, is taken off at the outer portion of the cylinder, thus providing the required pressure. The increased concentration mixture, representing dense brine, is not flowed to an outer radial point but is discharged at an inner radius of the rotating structure. This limits the ability to prevent the buildup of dense materials of the membranes which cause stagnation. This patent also uses the cylindrical membrane configuration with its limited surface area and replacement difficulties.
U.S. Pat. No. 3,567,030, "Reverse Osmosis Apparatus," by R. J. Loeffler and H. M. Bradbury has an excellent general description of the reverse osmosis process using centrifugally accelerated fluid mixtures. The rotating structure, of itself, provides the desired increased pressure for reverse osmosis, without requiring additional pumping. As with the previous patents, the cylindrical membrane configuration is used. The increased pressure brine solution flows radially inward through the membranes toward the axis of the rotating structure. This flow thus opposes the outward radial flow of the enriched brine. The enriched brine is removed from a region near the outer radius of the rotating structure.
U.S. Pat. No. 3,669,879, "Fluid Separation Apparatus and Method," issued to L. P. Berriman uses a cylindrical configuration similar to that described by Huntington. This patent describes a nozzle arrangement whereby the expelled desalinated water helps to rotate the basket. It also describes a variety of membrane configurations which fit within a basket shaped configuration in the rotating cylinder. These configurations utilize both cylindrical and radial membrane formations. The concentrated brine is exited at the outer radius of the rotating structure.